1. Field of the Invention
The present invention relates to a process for removing contaminants from thermally cracked waste oil, e.g. waste lubricating oil.
2. Description of the Prior Art
Numerous processes have been developed for producing diesel fuel from waste lubricating oil, referred to hereinafter as "used motor oil", by thermal cracking. However, the thermal cracking of used motor oils has serious problems with regard to product quality. Oil produced by thermal cracking of used motor oil has a strong odour, precipitates tar and darkens with time. All of these contribute to reduced marketability of the diesel fuel.
It is generally known that thermal cracking produces olefin-rich oil which rapidly changes colour and composition due to oxidation and polymerization reactions. This is described for instance in Balts, B. D. and Fathoni, A. Z., "A literature review on fuel stability studies with particular emphasis on diesel oil", Energy & Fuels, vol. 5, 2, 1991. That paper shows that cracked distillate tends to deteriorate more rapidly than straight run distillate during storage and that cracked products induce gum and sediment formation in fuels. Oxidation, auto-oxidation and chemical reactions, such as polymerization involving unsaturated hydrocarbons and/or reactive organic compounds of sulphur, nitrogen and oxygen present in the fuel are thought to be the major cause of gum or sediment formation.
Methanol extractions are described by Wechter, M. A. and Hardy, D. R., "The isolation of precursors responsible for insolubles formation in mid-distillate diesel fuels", Fuel Science and Technology Int'l, 7(4), 423-441 (1989). This article shows that when mid-distillate diesel fuels are extracted using methanol, insolubles in the fuel are reduced dramatically, with more than 95% of the fuel insolubles being soluble in methanol.
Sharma, Y. K, and Agrawal, K. M., "Influence of methanol extraction on the stability of middle distillate fuels", Fuel, 73(2), 269-271 (1994) also teaches about extractions with methanol. This article shows that when middle distillate fuels are extracted with methanol, the fuels form less insolubles, i.e. insolubles precursors are effectively removed.
Arganbright et al., U.S. Pat. No. 5,446,231, issued Aug. 29, 1995, describes a method for removing nitrile contaminants from C.sub.5 streams in which methanol may be used as a solvent. Darian et al., in U.S. Pat. No. 4,746,420, issued May 24, 1988, have also found that methanol may be used as a co-solvent in the removal of nitrogen compounds from diesel oils.
It is the object of the present invention to find an improved process for the removal of contaminants from thermally cracked waste oil, such as used motor oil.